Various analytical procedures and devices are commonly employed to determine the presence and/or concentration of analytes that may be present in a test sample. In some cases, the mere presence of an analyte may, for example, indicate the existence of tissue or organ damage. Likewise, abnormal concentrations of an analyte may indicate infection, such as a bacterial or viral infection. One conventional technique for detecting the presence of an enzymatic analyte is described in U.S. Pat. No. 6,348,319 to Braach-Maksvytis, et al. Braach-Maksvytis, et al. functions by sensing the digestion of a substrate by the enzyme. For example, FIG. 1 of Braach-Maksvytis. et al. illustrates a device 10 that includes a first zone 11 and a second zone 12. The first zone 11 is provided with polymer beads 13 (carrier) linked to streptavidin 14 (reporter) via a peptide linker 15 that is cleavable by a protease 16. Upon addition of the protease 16, the streptavidin 14 is released and passes to the second zone 12, which includes a biosensor membrane 17 that detects the presence of streptavidin through a change in the impedance of the membrane. (Col. 5, II. 25-30). Unfortunately, however, techniques such as described by Braach-Maksvytis, et al., are far too complex and cost prohibitive for certain types of applications, such as those requiring a relatively quick diagnosis by a patient (self-diagnosis or with the aid of medical personnel).
As a result, assays have been developed that are more ‘user friendly’. For instance, assays have been developed that utilize indicators susceptible to a visible or detectable change upon action of an analyte. If the indicator undergoes a detectable change, e.g., a color change, the user may be confident that the analyte is present in the test sample. For example, U.S. Pat. No. 5,409,664 to Allen describes an assay device including an amine-functionalized bibulous assay strip including a signal producing system. Specifically, the reagent strip is impregnated with one or more members of the detectable signal reagent system. For example, when determining the amount of cholesterol in a sample, one could have cholesterol esterase in a first zone, cholesterol oxidase in a second zone, and horseradish peroxidase in a third zone.
Despite such improvements, assays still present numerous difficulties. For instance, small molecular indicators usually work well in wet chemistry applications, in which a test sample is combined with a reagent-containing solution, but they are not equally suitable for use in dry chemistry applications due to, for instance, inability to maintain the dry small molecule indicators in an active state and at a specific location on a device during shipping and handling prior to use. However, such dry chemistry applications are preferred due to there relative simplicity, speed of detection, and low cost.
As such, a need currently exists for improved assay devices, in particular for assay devices directed to dry chemistry applications.